Polyurethanes are polymers that contain a significant number of urethane linkages within its molecular chain. The urethane linkages are formed by the condensation reaction of an isocyanate and an active hydrogen compound. The reaction product of the condensation reaction depends upon the structure of the active hydrogen compound. The reaction of an isocyanate with a hydroxyl group forms urethane, while the reaction with a primary amine forms a substituted urea. The reaction of isocyanates with carboxylic acids, which produces amides, and the reaction of isocyanates with water, which produces amines, both result in the liberation of carbon dioxide gas.
Thousands of different polyurethanes can be made from the array of commercially available isocyanate and active hydrogen compounds. They are typically divided into two categories, namely one-component systems and two-component systems.
The most common type of one-component system is a specially formulated prepolymer that is allowed to moisture cure with water vapor in the atmosphere. Other one-component systems are allowed to vapor cure under an amine atmosphere. One-component systems can also be produced by chemically blocking isocyanate groups from reacting with active hydrogen compounds. Heat is applied to break the bonds of the blocking groups, allowing the polyurethane to cure.
Two-component systems may take one of two forms. The first, known as a one-shot system, comprises a resin component, which is a blend of an active hydrogen compound or mixture of active hydrogen compounds with catalysts, crosslinkers, surfactants, flame retardants, plasticisers, fillers, etc., that is mixed with an isocyanate component, made up of only polyisocyanate. The second form of two-component system is known as a prepolymer system. First, a prepolymer is formed by reacting a portion of the polyisocyanate with an active hydrogen compound, which results in a polyisocyanate rich component, sometimes referred to as the B component. The prepolymer is then reacted with a chain extender, sometimes referred to as the A component, which is a blend of an active hydrogen compound or mixture of active hydrogen compounds with catalysts, crosslinkers, surfactants, flame retardants, plasticisers, fillers, etc. The chain extender increases the molecular weight of the prepolymer molecules and creates the final polyurethane polymer.
The conventional application of polyurethane as an athletic surface typically comprises a composite of rubber particles bound with a two-component polyurethane system to form a base mat. The two components of the polyurethane system are generally mixed on site along with the rubber particles to form the composite surface. The base mat may be used alone, or it may be enhanced by the addition of a structural spray comprising a mixture of polyurethane and rubber sprayed on top the base mat creating a textured surface. The base mat can also be coated with a flood coat of polyurethane and rubber. Alternatively, a full-pour system may be employed in which multiple layers are mixed and poured in place.
Regardless of the method employed, the formulation of the polyurethane system is of critical importance. Since the polyurethane is mixed on site, the formulation must be simple to use. The ideal formulation would comprise substantially equivalent volumes of the two components (a 1:1 system), which would reduce mixing errors. The formulation should also have an adequate gel time to permit the proper preparation of the surface, i.e. mixing, pouring, raking, etc. Further, adequate curing of the polyurethane should occur over a relatively broad range of ambient temperatures and relative humidity, and should be sufficient to ensure that the proper physical properties of the surface or attained. Finally, the formulation should minimize deleterious effects to the persons preparing the surface and to the environment. While conventional polyurethane systems may have addressed some of these issues individually, a system has not been formulated that addresses all of these issues and provides an athletic surface having acceptable physical properties.